On the Mass-Donating Star in GRS 1915+105

We present an analysis of constraints on the companion mass for the Galactic microquasar GRS 1915+105. Using the known inclination angle and stability of the jet axis, we can rule out massive ($M_s \ga 3 M_{\sun}$) companions with orbital axes not aligned with the jet axis. For an aligned orbital axis, we constrain the ratio of the stellar radius to the binary semi-major axis to be $R_s/a <0.342$, based on the lack of X-ray eclipses. We then show that these constraints together with the X-ray luminosity, approximate black hole mass, and observations of the X-ray absorbing column density towards GRS 1915+105 rule out accretion via stellar wind from a massive companion, implying that accretion occurs via Roche-lobe overflow. In that case, the constraint on $R_s/a$ rules out any companion with $M_s > 19.4 M_{\sun}$ for a black hole mass of $M_{BH} \simeq 30 M_{\sun}$. The lack of significant X-ray reprocessing in the infrared from GRS 1915+105 also provides constraints on the mass and temperature range of a Roche-lobe-filling mass donor.